Page:VCH Herefordshire 1.djvu/170

 A HISTORY OF HEREFORDSHIRE of harm. Their presence in great numbers may be a sign of untidiness or neglect. But only a few, even of the widely distributed species, appear to form large associations in any one locality. Their favourite resorts under moist stones and potsherds, rotting leaves and mouldering bark, are not very consistent with an epicurean appetite ; and if they are occasionally found with the plundering wasp in a half excavated peach or a decaying apple, it is not likely that they began the depredation in which they are sharing. At any rate Herefordshire, famous for fruits, has recorded no verdict against them, and scientifically has taken no notice of them at all. The county can be credited certainly with five species, probably with several others. The five in question all belong to the tribe Oniscidea, and to one family in that tribe, the Oniscidae. Here as usual there are two pairs of antennae, but the first pair, besides being only three-jointed, are inconspicuous in size and position. The pleon is not consolidated as in Asellus, but exhibits six articulated segments. The sixth of these is very small, yet no doubt it is composite, the basal part which carries the uropods representing the true sixth segment, while the narrowed apex represents the seventh segment or telson. This composite piece, which in some of the Isopoda attains a great size, may be called the telsonic segment. The general organization makes it sufficiently clear that these shrimps of the garden and the woodland are derived from aquatic progenitors. It may be asked what inducement they had to forsake their natural element. Possibly hostile pursuit gave the impulse. Just as in naval warfare a sloop might find refuge in shoals and intricate recesses of the coast whither frigate and man-of-war cannot follow it, small crustaceans may have beaten a retreat from depth to shallow, from shallow to spray-moistened shore, and so on by slow degrees to situations merely humid or completely dry. In this progression it would not be surprising if the branchial pleopods derived from marine ancestors experienced some modifications in favour of subaerial respiration. Whether it be surprising or not, what we find is this : In some of the genera of the Oniscidae the pleopods are tracheate — that is, they are furnished with pseudo-tracheae, an arrangement approximating to the proper tracheae of insects. Among the genera with which we are here concerned, Oniscus, Philoscia, and Platyarthrus are non-tracheate, Porcellio and Metopomrthus are tracheate. The second antennae help to a further and to some extent easier discrimination. These organs have a five-jointed peduncle, carrying a terminal flagellum or lash. In many crustaceans this flagellum being many-jointed, long, and flexible, is obviously whip-like. In this family, however, its joints are very few, but none the less an aid in classification. Thus in Oniscus and Philoscia the flagellum is three-jointed, while it is only two-jointed in the other three genera. But Oniscus and Philoscia can be readily separated by another obvious character, because in the latter genus the pleon is abruptly narrower than the peraeon or middle body, whereas in Oniscus the outline of the two parts is continuous. The same distinction of a narrowed pleon separates our British species of Metopomrthus from Porcellio. The most abundant of our British species, and the largest of the five here under discussion, is Oniscus asellus, Linn. It was obtained at Ludford and at Berrington under stones, and occurs practically all over England. It is extremely prolific. In the second antennae the first and third joints of the flagellum are each longer than the second, but in young specimens both the ii8